1. Field of the Invention
The present invention relates to a technology of locating a moving object to track down a location of the moving object.
2. Description of the Related Art
As a method for measuring position of moving object, a global positioning system (GPS) has been widely utilized. Although positioning by GPS alone has an accuracy of about 10 meters (m) and GPS is practical as a positioning technology for navigation services to guide a driver, its accuracy is insufficient as the method for high-level vehicle control such as intervention control or automatic vehicle control.
In addition, although use of a GPS (e.g., real time kinematic (RTK) and virtual reference station-real time kinematic (VRS-RTK), etc.) enables accuracy within centimeters, however, problems such as high cost, deterioration of accuracy resulting from an effect of a reflected wave, and long recovery time to original status after the electromagnetic wave is disrupted such as by a tunnel, etc., make use of difficult to use this method on actual road.
Another positioning method takes photographs of multiple markers using a camera mounted on a moving object and executes position-locating using an angle that is formed between two lines from the camera to the markers. FIG. 16 illustrates such a position-locating method that uses an angle formed by two lines directing from camera mounted on a moving object to the markers of which photographs were taken by the camera.
As shown in FIG. 16, if positions of multiple markers P1, P2, P3, . . . are known, two markers Pi, Pj (i, j=1, 2, 3, . . . , here i≠j) observed by camera and the moving object exists on the same circle (marker circle Eij) having a center position Oij expressed by equation 1.1 and a radius rij expressed by equation 1.2.
                              O          ij                =                              (                                                                                P                    ix                                    +                                      P                    jx                                                  2                            ,                                                                    P                    iy                                    +                                      P                    jy                                                  2                                      )                    -                                    1                              tan                ⁢                                                                  ⁢                                  θ                  ij                                                      ⁢                          (                                                                                          P                      jy                                        -                                          P                      iy                                                        2                                ,                                  -                                                                                    P                        jx                                            -                                              P                        ix                                                              2                                                              )                                                          (        1.1        )                                          r          ij                =                              1                          2              ⁢                                                          ⁢              sin              ⁢                                                          ⁢                              θ                ij                                              ⁢                                                                      (                                                            P                      jx                                        -                                          P                      ix                                                        )                                2                            ⁢                                                (                                                            P                      jy                                        -                                          P                      iy                                                        )                                2                                                                        (        1.2        )            
In equation 1.2, θij is an inter-marker angle that is formed by the lines directed from camera to two markers Pi and Pj. θij is calculated using photographic images taken from the moving object origin Om When more than 3 markers are detected, since it is possible to obtain multiple marker circles Eij from equations 1.1 and 1.2, it is possible to obtain the moving object position continuously by calculating an intersecting point of these marker circles Eij (as shown in FIG. 16, E12 and E23).
Although the method is able to obtain the position of the moving object with high accuracy, a method to measure angle of a line directed to the marker having a measuring range of 360 degrees in reference to the moving object origin Om is required. Therefore, use of a special all-direction camera having the optical center at the moving object origin Om is required.
FIG. 17 is a diagram showing an example of images taken by an optical system of an all-direction camera and the relevant optical system. As shown FIG. 17, a projection plane f is formed by a side surface of a cylinder with a circle line going through the optical center as the center axis Y. The projected image is recorded with its horizontal axis in correspondence with an azimuthal angle θ. Thus, it is possible to calculate easily the marker azimuthal angle and inter-marker angle (as shown in FIGS. 17, θ12 and θ23) from the horizontal pixel position of the projected image of the markers.
In order to realize such an all-direction camera, for example in Japanese Patent Application Laid-Open Publication No. H11-83480, a mechanism that rotates the camera 360 degrees about the optical center as an axis is equipped, and in Japanese Patent Application Laid-Open Publication No. 2000-337887, an optical system including a special reflector plate and lens is utilized.
Further, without using such an all-direction camera, there also exist various types of systems and products that create a panorama image with a 360 degree field of view by use of a plurality of cameras. FIG. 18 shows an example of an image taken when a plurality cameras CM1 to CM3 is mounted on a vehicle. As shown in FIG. 18, each camera CM1 to CM3 is able to take an image of marker P1, P2, P3, . . . that is located within the field of vision. By combining cameras CM1 to CM3 multiple times, it is possible to generate the peripheral view (For example, see Japanese Patent Application Laid-Open Publication No. 2006-54662).
Devices employing conventional technology using the described all-direction camera become mechanically complex, expensive in cost and large in size. In addition, when mounting the all-direction camera on a moving object, it is necessary to place it in a location such that a 360 degree field of vision is obtained. For moving objects such as vehicles, camera mounting is typically limited to the roof of the vehicle, i.e., installation flexibility is significantly low. Especially for automobiles, since the exterior design is very important, the lack of flexibility has been a serious problem that imposes a severe limitation on vehicle style relative to camera mounting.
For other conventional technologies employing a plurality of cameras, problems related to deviations from the optical center of each camera CM1 to CM3 have not been actively addressed. For example, as shown in FIG. 18, although direction of the markers P1, P2, and P3 photographed by the camera CM1 to CM3 can be measured based on unique coordinate system of camera CM1 to CM3 (shown by circle in FIG. 18), there has been a problem that it was impossible to measure direction of P1, P2, and P3 in case optical center is taken as moving object origin Om because of offset of optical center of camera CM1 to CM3, or even if it is not impossible, accuracy of measurement is insufficient (indicated by reference character “x” in FIG. 18).